A multi-master bus is a computer bus

Computer bus

In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...

 in which there are multiple bus master

Bus mastering

In computing, bus mastering is a feature supported by many bus architectures that enables a device connected to the bus to initiate transactions...

 nodes present on the bus.
This is used when multiple nodes on the bus must/need to initiate transfer.
For example, direct memory access

Direct memory access

Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....

 (DMA) is used to transfer data between peripherals and memory without the need to use the central processing unit

Central processing unit

The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

 (CPU).

Some buses like I²C

I²C

I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. Since the mid 1990s, several competitors I²C ("i-squared cee" or "i-two cee"; Inter-Integrated Circuit;...

use multi-mastering inherently to allow any node to initiate a transfer with another node.